Metal-organic frameworks for sorption and sensing applications

ABSTRACT

Metal-organic frameworks for capturing one or more of SO2, CO2, and H2O are disclosed herein. Non-limiting examples of metal-organic frameworks include NbOFFIVE-1-Ni and AIFFIVE-1-Ni, among others. The metal-organic frameworks can be used in applications for removing and/or sensing one or more of SO2, CO2, and H2O from a fluid composition or an environment, either of which can proceed under dry or humid conditions and/or at room temperature.

BACKGROUND

Global warming and other environmental/ecological issues have forced oursociety to adopt stringent rules for industrial waste and be on thelookout for ways to improve indoor and outdoor air quality, including inindustrial sites. One major form of industrial waste with an adverseeffect on the environment is flue gas. Flue gas generated by largeindustries and power plants resulting from burning fossil fuel containsCO₂ (at a low percent concentration), SO₂ (500-2000 ppm), NO₂ (few ppm),water vapor, and nitrogen (as the dominant gas). Although theconcentration of SO₂ in flue gas feed is low, it could be poisonous formost liquid- and/or solid-state-based CO₂ separating agents. Therefore,the removal of SO₂ from flue gas is of prime importance. Current SO₂removal technology involves the irreversible acid-base reaction of SO₂with CaO to form CaSO₃. The main drawback of this technology is itsrelatively low removal efficiency (<90%) associated with an almostimpossible regeneration step due to its extremely energy-intensive cost.Therefore, cyclable physical sorption technology is perceived as analternate approach. Hence, identification of an adsorbent that canefficiently capture SO₂ at low concentration (<500 ppm) is crucial.

When SO₂ is not controlled and is emitted into the atmosphere, it hasadverse effects on the environment, such as acid rain, and must bemonitored. Therefore, it is necessary to find efficient solutions tosense SO₂ at ppm level (above 25 ppm) in both dry and humid conditions.Recently, there has been a considerable effort to develop SO₂ sensingdevices based on metal oxides (such as SnO₂, WO₃, and TiO₂) due to theirexcellent sensitivity, selectivity, response time, and recovery time.However, most of the semiconductor-based SO₂ sensors were reported torequire high temperatures (200-600° C.), leading to high levels of powerconsumption. There is therefore a need for gas sensors that operate atroom temperature (RT), which would be an important parameter andinvaluable milestone for developing alternate materials suitable fordetecting SO₂.

In addition, maintaining safe levels of CO₂ and humidity in indoorenvironments or confined spaces is a top priority but often also a majorchallenge for environmental technologies. Consequently, major researchefforts have recently been devoted to developing new technologies andprocesses that can effectively detect and measure indoor levels of CO₂and relative humidity (RH). Technologies currently available for thedetection of CO₂ are based on either nondispersive infrared (NDIR)sensors or chemical CO₂ gas sensors. These technologies present manydrawbacks, such as elevated working temperatures (300-800° C.),prohibitive costs, and relatively shortened lifetimes. Transductiontechniques, including acoustic, resistance, magnetic, resonance,optical, impedance, delay line, capacitance, thermal, and quartz crystalmicrobalance (QCM) techniques, have allowed the development of efficienthumidity sensors. However, the presence of large amounts of CO₂ (foundmainly in confined spaces) can alter the measure of RH levels bysensors. It is therefore imperative to develop robust and inexpensivesensors with the ability of simultaneously detecting CO₂ and H₂O in aprecise and reliable manner.

SUMMARY

In general, embodiments of the present disclosure relate tometal-organic frameworks for capturing and/or sensing one or more ofSO₂, CO₂, and H₂O, methods of capturing and/or sensing one or more ofSO₂, CO₂, and H₂O using the metal-organic frameworks, and the like. Inone aspect, the metal-organic frameworks are selective for one of SO₂,CO₂, and H₂O. In another aspect, the metal-organic frameworksconcurrently (e.g., simultaneously) or sequentially capture and/or sensetwo or more of SO₂, CO₂, and H₂O.

Accordingly, embodiments of the present disclosure describe methods ofcapturing chemical species comprising contacting a metal-organicframework with a fluid composition including one or more of SO₂, CO₂,and H₂O, wherein the metal-organic framework comprises a square gridpillared by an inorganic building block, wherein the square grid isNi(pyrazine)₂ and the inorganic building block is selected from[NbOF₅]²⁻ or [AlF₅(H₂O)]²⁻; and sorbing one or more of SO₂, CO₂, and H₂Ofrom the fluid composition on the metal-organic framework.

In some embodiments, the fluid composition includes SO₂ at aconcentration in the range of about 25 ppm to about 500 ppm. In someembodiments, the fluid composition includes SO₂ and CO₂, and SO₂ ispreferentially sorbed over CO₂ on the metal-organic framework. In someembodiments, the fluid composition includes SO₂ and CO₂, and SO₂ and CO₂are both sorbed on the metal-organic framework. In some embodiments, SO₂and CO₂ are sorbed about simultaneously on the metal-organic framework.In some embodiments, the fluid composition includes CO₂ at aconcentration in the range of about 400 ppm to about 5000 ppm. In someembodiments, the fluid composition includes CO₂ and H₂O, and CO₂ ispreferentially sorbed over H₂O on the metal-organic framework. In someembodiments, the fluid composition includes CO₂ and H₂O, and CO₂ and H₂Oare both sorbed on the metal-organic framework. In some embodiments, theCO₂ and H₂O are sorbed about simultaneously on the metal-organicframework. In some embodiments, the sorbing proceeds at about roomtemperature.

Embodiments of the present disclosure describe methods of capturingchemical species using NbOFFIVE-1-Ni comprising contacting ametal-organic framework with a fluid composition including at least SO₂and CO₂, wherein the metal-organic framework is characterized by thechemical formula NiNbOF₅(pyrazine)₂·x(solv); sorbing one or more of SO₂and CO₂ on the metal-organic framework; and optionally regenerating themetal-organic framework.

Embodiments of the present disclosure describe methods of capturingchemical species using AlFFIVE-1-Ni comprising contacting ametal-organic framework with a fluid composition including at least SO₂and CO₂, wherein the metal-organic framework is characterized by thechemical formula NiAlF₅(H₂O)(pyrazine)₂·x(solv); sorbing one or more ofSO₂ and CO₂ on the metal-organic framework; and optionally regeneratingthe metal-organic framework.

Embodiments of the present disclosure describe methods of detecting oneor more analytes comprising exposing a sensor to an environmentcontaining one or more of SO₂, CO₂, and H₂O, wherein the sensor includesa layer of a metal-organic framework as a sensing layer, wherein themetal-organic framework comprises a square grid pillared by an inorganicbuilding block, wherein the square grid is Ni(pyrazine)₂ and theinorganic building block is selected from [NbOF₅]²⁻ or [AlF₅(H₂O)]²⁻;and detecting a presence of the SO₂, CO₂, H₂O, or a combination thereofin the environment using the sensor.

In some embodiments, the detecting proceeds at about room temperature.In some embodiments, the detecting includes detecting SO₂ optionally inthe presence of H₂O. In some embodiments, the detecting includesdetecting between 25 ppm SO₂ to about 500 ppm SO₂. In some embodiments,the detecting includes detecting CO₂ optionally in the presence of H₂O.In some embodiments, the detecting includes detecting between about 400ppm of CO₂ and 5000 ppm of CO₂. In some embodiments, the detectingincludes detecting H₂O optionally in the presence of CO₂. In someembodiments, the detecting includes detecting relative humidity levelsin the environment below about 40% RH and/or greater than about 60% RH.In some embodiments, the sensor is a capacitive sensor comprising aninterdigitated electrode, wherein the sensing layer is deposited on theinterdigitated electrode of the capacitive sensor, wherein the presenceof one or more of SO₂, CO₂, and H₂O is detected by measuring a change incapacitance in the sensing layer. In some embodiments, the sensor is aQCM sensor comprising an electrode, wherein the sensing layer isdeposited on the electrode of the QCM, wherein the presence of one ormore of SO₂, CO₂, and H₂O is detected by measuring a change in resonancefrequency in the sensing layer.

Embodiments of the present disclosure describe methods of sensing usingNbOFFIVE-1-Ni comprising exposing a sensor to an environment containingat least SO₂, wherein the sensor includes a layer of a metal-organicframework as a sensing layer; wherein the metal-organic framework ischaracterized by NiNbOF₅(pyrazine)₂·x(solv); detecting a presence of SO₂in the environment using the sensor; and optionally regenerating thesensor.

Embodiments of the present disclosure describe methods of sensing usingAlFFIVE-1-Ni comprising exposing a sensor to an environment containingat least SO₂, wherein the sensor includes a layer of a metal-organicframework as a sensing layer; wherein the metal-organic framework ischaracterized by NiAlF₅(H₂O)(pyrazine)₂·x(solv); detecting a presence ofSO₂ in the environment using the sensor; and optionally regenerating thesensor.

The details of one or more examples are set forth in the descriptionbelow. Other features, objects, and advantages will be apparent from thedescription and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

This written disclosure describes illustrative embodiments that arenon-limiting and non-exhaustive. In the drawings, which are notnecessarily drawn to scale, like numerals describe substantially similarcomponents throughout the several views. Like numerals having differentletter suffixes represent different instances of substantially similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

Reference is made to illustrative embodiments that are depicted in thefigures, in which:

FIG. 1 is a flowchart of a method of capturing chemical species,according to one or more embodiments of the present disclosure.

FIG. 2 is a flowchart of a method of capturing chemical species usingNbOFFIVE-1-Ni, according to one or more embodiments of the presentdisclosure.

FIG. 3 is a flowchart of a method of capturing chemical species usingAlFFIVE-1-Ni, according to one or more embodiments of the presentdisclosure.

FIG. 4 is a flowchart of a method of sensing, according to one or moreembodiments of the present disclosure.

FIG. 5 is a flowchart of a method of sensing using NbOFFIVE-1-Ni,according to one or more embodiments of the present disclosure.

FIG. 6 is a flowchart of a method of AlFFIVE-1-Ni, according to one ormore embodiments of the present disclosure.

FIG. 7 is a schematic diagram showing the column breakthrough set-up,according to one or more embodiments of the present disclosure.

FIG. 8 is a schematic diagram showing gas flow control and dilutionsystem and sensor measurement set-up, according to one or moreembodiments of the present disclosure.

FIGS. 9A-9C is a schematic diagram showing crystal structure details forNbOFFIVE-1-Ni and AlFFIVE-1-Ni: (a) Square grid resulting from theconnection of Ni²⁺ cations and pyrazine that are further pillared byeither (b) (AlF₅ (H₂O))²⁻ pillar or (c) (NbOF₅)²⁻ pillar, according toone or more embodiments of the present disclosure.

FIG. 10 is a graphical view of SO₂ isotherm for NbOFFIVE-1-Ni at 25° C.after 105° C. activation, according to one or more embodiments of thepresent disclosure.

FIGS. 11A-11D are schematic diagrams showing local views for theDFT-geometry optimized guest loaded-crystal structures: (a) SO₂ and (b)CO₂— loaded in NbOFFIVE-1-Ni, (c) SO₂ and (d) CO₂— loaded inAlFFIVE-1-Ni (Color code: Aluminum (pink), niobium (green), nickel(steel blue), fluorine (light green), nitrogen (blue), carbon (gray),hydrogen (white), oxygen (red), and sulfur (yellow)), according to oneor more embodiments of the present disclosure.

FIGS. 12A-12D are graphical views showing NbOFFIVE-1-Ni adsorptioncolumn breakthrough experiments for (a) SO₂/N₂: 7/93 mixture (10 cc/min,flow rate), (b) SO₂/N₂: 0.05/99.95 mixture (40 cc/min, flow rate), and(c) SO₂/CO₂/N₂: 0.05/10/89.95 mixture (25 cc/min, flow rate); (d)Temperature-programed desorption after initial adsorption in the columnusing a mixture akin to flue gas (SO₂/CO₂/N₂: 0.05/10/89.95), suggestingan adsorbed phase composition dominated by CO₂, according to one or moreembodiments of the present disclosure.

FIG. 13 is a graphical view showing NbOFFIVE-1-Ni breakthroughexperiments for 4% CO₂, 4% SO₂ (balance N₂), according to one or moreembodiments of the present disclosure.

FIG. 14 is a graphical view of SO₂ isotherm for AlFFIVE-1-Ni at 25° C.after 105° C. activation, according to one or more embodiments of thepresent disclosure.

FIGS. 15A-15D are graphical views showing AlFFIVE-1-Ni breakthroughexperiments for (a) SO₂/N₂:7/93 (10 cc/min, flow rate), (b)SO₂/N₂:0.05/99.95 mixture (40 cc/min, flow rate), and (c)SO₂/CO₂/N₂:0.05/10/89.95 (40 cc/min, flow rate); (d) TPD experimentsuggests a considerable amount of SO₂ along with CO₂ as adsorbed phaseafter a breakthrough experiment with 500 ppm SO₂ in the presence of 10%CO₂ and balance N₂, according to one or more embodiments of the presentdisclosure.

FIG. 16 is a graphical view showing AlFFIVE-1-Ni breakthroughexperiments for 7% SO₂ (balance N₂) and optimization of regenerationtemperature, according to one or more embodiments of the presentdisclosure.

FIG. 17 is a graphical view showing TPD analysis of adsorbed phase forAlFFIVE-1-Ni after breakthrough experiments with 500 ppm SO₂ (balanceN₂), according to one or more embodiments of the present disclosure.

FIGS. 18A-18B are SEM micrographs at a high and low magnification of (a)NbOFFIVE-1-Ni and (b) AlFFIVE-1-Ni thin films coated on the goldelectrode of QCM, according to one or more embodiments of the presentdisclosure.

FIGS. 19A-19B are graphical views of final Le Bail profile refinementwith observed (black line), calculated (red point), and difference (blueline) profiles of X-ray of diffraction data, vertical green bars arerelated to the calculated Bragg reflection positions: (a) NbOFFIVE-1-Ni(Rp=0.074, Rwp=0.079, Rexp=0.031, χ2=6.73). (b) AlFFIVE-1-Ni (Rp=0.086,Rwp=0.108, Rexp=0.022,)(2=23.1), according to one or more embodiments ofthe present disclosure.

FIG. 20 is a graphical view showing frequency shift comparison as afunction of the SO₂ concentration for the uncoated and NbOFFIVE-1-Ni orAlFFIVE-1-Ni coated QCM sensors, according to one or more embodiments ofthe present disclosure.

FIGS. 21A-21B are graphical views showing variation of the peakresonance frequency (a) NbOFFIVE-1-Ni (b) AlFFIVE-1-Ni, in response tothe introduction of various concentrations of dry SO₂, according to oneor more embodiments of the present disclosure.

FIG. 22 is a graphical view showing plots of sensors' responses as afunction of SO₂ concentration in synthetic air, according to one or moreembodiments of the present disclosure.

FIGS. 23A-23B are graphical views showing variation of the peakresonance frequency (a) NbOFFIVE-1-Ni (b) AlFFIVE-1-Ni, in response tothe introduction of various concentrations of humid SO₂, according toone or more embodiments of the present disclosure.

FIG. 24 is a graphical view of temperature-programed desorption (TPD) ofSO₂; during typical experiment materials were saturated with water at60% RH by flowing humid He, according to one or more embodiments of thepresent disclosure.

FIGS. 25A-25B are graphical views of long-term stability property of the(a) NbOFFIVE-1-Ni and (b) AlFFIVE-1-Ni sensors exposed to 50, 100, and157 ppm SO₂ gas, according to one or more embodiments of the presentdisclosure.

FIG. 26 is a schematic diagram illustrating the selective removal andsensing of SO₂ from air using fluorinated MOF Platform NbOFFIVE-1-Ni andAlFFIVE-1-Ni, according to one or more embodiments of the presentdisclosure.

FIGS. 27A-27B are SEM images of (a) NBOFFIVE-1-Ni and (B) AlFFIVE-1-NIthin films, according to one or more embodiments of the presentdisclosure.

FIGS. 28A-28B are graphical views of final Le Bail profile refinementwith observed (black line), calculated (red point), and difference (blueline) profiles of X-ray of diffraction data; vertical green bars arerelated to the calculated Bragg reflection positions, where (a)NbOFFIVE-1-Ni (R_(p)=0.074, R_(wp)=0.079, R_(exp)=0.031, χ²=6.73). (b)AlFFIVE-1-Ni (R_(p)=0.086, R_(wp)=0.108, R_(exp)=0.022, χ²=23.1),according to one or more embodiments of the present disclosure.

FIG. 29 is a graphical view of a voltage to temperature calibrationcurve, according to one or more embodiments of the present disclosure.

FIGS. 30A-30D are graphical views showing the capacitive sensor responsefor (a) NbOFFIVE-1-Ni and (b) AlFFIVE-1-Ni and the comparison offrequency shifts as a function of the CO₂ concentration for (c)NbOFFIVE-1-Ni and (d) AlFFIVE-1-Ni, according to one or more embodimentsof the present disclosure.

FIGS. 31A-31D are graphical views showing the stability detection of CO₂upon exposure to 60% RH of the NbOFFIVE-1-Ni and AlFFIVE-1-Ni filmsbased on (a-b) QCM and (c-d) IDE sensors, according to one or moreembodiments of the present disclosure.

FIGS. 32A-32D are graphical views showing the variation in capacitancewith relative humidity (RH) change in (a) NbOFFIVE-1-Ni and (b)AlFFIVE-1-Ni and frequency shift as a function of RH for QCM coated with(c) NbOFFIVE-1-Ni and (d) AlFFIVE-1-Ni, according to one or moreembodiments of the present disclosure.

FIGS. 33A-33D are graphical views showing the stability detection of H₂Oupon exposure to 500 ppm CO₂ of the NbOFFIVE-1-Ni and AlFFIVE-1-Ni filmsbased on (a-b) QCM and (c-d) IDE sensors, according to one or moreembodiments of the present disclosure.

FIGS. 34A-34B are graphical views showing long-term stability propertiesof (a) H₂O upon prior exposure to 500 ppm CO₂ of the AlFFIVE-1-Ni/IDEsensor and (b) CO₂ upon prior exposure to 60% RH of theNbOFFIVE-1-Ni/QCM sensor, according to one or more embodiments of thepresent disclosure.

DETAILED DESCRIPTION

Metal-organic frameworks for capturing one or more of SO₂, CO₂, and H₂Oare disclosed herein. Non-limiting examples of metal-organic frameworksinclude NbOFFIVE-1-Ni and AlFFIVE-1-Ni, among others. The metal-organicframeworks can be used in applications for removing and/or sensing oneor more of SO₂, CO₂, and H₂O from a fluid composition or an environment,either of which can proceed under dry or humid conditions and/or at roomtemperature. For example, the metal-organic frameworks can be used assorbents for removing SO₂ from flue gas, or the metal-organic frameworkscan be incorporated into QCM- or IDE-based sensors as the sensing layerfor detecting and/or measuring the presence of one or more of SO₂, CO₂,and H₂O. Such sensors can detect concentrations as low as 25 ppm to 500ppm SO₂, 400 ppm to 5000 ppm CO₂, and relative humidity levels in anenvironment below 25% RH and/or above 65% RH, all of which isunprecedented. In either application, the sorption can be reversiblesuch that the metal-organic frameworks may be regenerated and reused.These features and others are described elsewhere herein.

Definitions

The terms recited below have been defined as described below. All otherterms and phrases in this disclosure shall be construed according totheir ordinary meaning as understood by one of skill in the art.

As used herein, “contacting” refers to the act of touching, makingcontact, or of bringing to close or immediate proximity, including atthe cellular or molecular level, for example, to bring about aphysiological reaction, a chemical reaction, or a physical change (e.g.,in solution, in a reaction mixture, in vitro, or in vivo). Contactingmay refer to bringing two or more components in proximity, such asphysically, chemically, electrically, or some combination thereof.Mixing is an example of contacting.

As used herein, “contacting” may, in addition or in the alternative,refer to, among other things, feeding, flowing, passing, injecting,introducing, and/or providing the fluid composition (e.g., a feed gas).

As used herein, “detecting” refers to determining a presence and/orconcentration of one or more chemical species.

As used herein, “exposing” refers to subjecting to conditions of anenvironment. For example, conditions of an environment may include,among other things, one or more of temperature, pressure, and chemicalspecies present in the environment. In addition or in the alternative,exposing refers to subjecting to objects present in an environment.

As used herein, “sorbing” refers to one or more of absorbing andadsorbing. Sorbing may include selective sorption, such as sorption of asingle species, subsequent sorption, such as sorption of a first speciesand then a second species (e.g., which may or may not replace the firstspecies), or simultaneous sorption, such as sorption of two or morespecies at about the same time. Capturing is an example of sorbing.

As used herein, “capturing” refers to the act of removing one or morechemical species from a bulk fluid composition (e.g., gas/vapor, liquid,and/or solid). For example, “capturing” may include, but is not limitedto, interacting, bonding, diffusing, adsorbing, absorbing, reacting, andsieving, whether chemically, electronically, electrostatically,physically, or kinetically driven.

FIG. 1 is a flowchart of a method of capturing chemical species,according to one or more embodiments of the present disclosure. As shownin FIG. 1 , the method 100 may comprise contacting 101 a metal-organicframework with a fluid composition including one or more of SO₂, CO₂,and H₂O; sorbing 102 one or more of SO₂, CO₂, and H₂O from the fluidcomposition on the metal-organic framework; and optionally regenerating103 the metal-organic framework.

The step 101 includes contacting a metal-organic framework with a fluidcomposition including at least one or more of SO₂, CO₂, and H₂O. Thecontacting may include bringing the metal-organic framework and fluidcomposition into physical contact, or immediate or close proximityExamples of the contacting may include, but are not limited to, one ormore of feeding, flowing, passing, pumping, and introducing. Thecontacting may proceed under any suitable conditions (e.g., temperature,pressure, etc.). For example, the contacting may proceed to or at atemperature ranging from about 0° C. to about 600° C. In manyembodiments, the contacting may proceed at or to a temperature less thanabout 200° C. In preferred embodiments, the contacting may proceed at orto a temperature of about 25° C. (e.g., about room temperature).

The metal-organic framework may include fluorinated metal-organicframeworks characterized by square grids and pillars. The metal-organicframework may include a pillar characterized by the formulaM_(b)F₅(O/H₂O), where M_(b) is Al³⁺ or Nb⁵⁺. The pillar may include aninorganic pillar or inorganic building block. In an embodiment, thepillar may be characterized by the chemical formula: (AlF₅(H₂O))²⁻. Inan embodiment, the pillar may be characterized by the chemical formula:(NbOF₅)²⁻. The metal-organic framework may include a square gridcharacterized by the formula (M_(b)(ligand)_(x)), where Ma is Ni and theligand is pyrazine. In an embodiment, the square grid may becharacterized by the formula (Ni(pyrazine)₂). The pillar and square gridmay assemble and/or associate to form a metal-organic frameworkcharacterized by one or more of the following chemical formulas:NiNbOF₅(pyrazine)₂·x(solv) (NbOFFIVE-1-Ni) andNiAlF₅(H₂O)(pyrazine)₂·x(solv) (AlFFIVE-1-Ni). For example, in anembodiment, the metal-organic framework may be characterized by thechemical formula: NiNbOF₅(pyrazine)₂·x(solv). In an embodiment, themetal-organic framework may be characterized by the chemical formula:NiAlF₅(H₂O)(pyrazine)₂·x(solv). The metal-organic frameworks may includea periodic array of open metal coordination sites and fluorine moietieswithin a contracted square-shaped one-dimensional channel. In anembodiment, the metal-organic frameworks may include AlFFIVE-1-Ni,wherein the AlFFIVE-1-Ni includes three pendant fluoride groups with afluoride-fluoride distance of about 3.613 Å and one potential open metalsite. In an embodiment, the metal-organic framework may includeNbOFFIVE-1-Ni, wherein the NbOFFIVE-1-Ni includes four pendant fluoridegroups with a fluoride-fluoride distance of about 3.210 and no openmetal site.

The fluid composition may be present in any phase. For example, thefluid composition may be present in one or more of a gas/vapor phase,liquid phase, and solid phase. In many embodiments, the fluidcomposition may be present in a gas/vapor phase. The fluid compositionmay include one or more of SO₂, CO₂, and water (e.g., as water vaporand/or moisture, humidity) and optionally one or more other chemicalspecies. In some embodiments, the fluid composition includes at leastSO₂, optionally CO₂ and optionally water, and optionally one or moreother chemical species. For example, in an embodiment, the fluidcomposition includes at least SO₂. In an embodiment, the fluidcomposition includes at least SO₂ and CO₂, and optionally water. In anembodiment, the fluid composition includes at least SO₂, CO₂, and water.In an embodiment, the fluid composition includes at least SO₂, CO₂, andwater, and one or more other chemical species. The one or more otherchemical species may include one or more of NO₂ and nitrogen. Forexample, in an embodiment, the fluid composition may be synthetic fluegas, which may include one or more of SO₂, CO₂, water vapor, NO₂, andnitrogen. In some embodiments, the fluid composition includes CO₂ andoptionally H₂O. For example, in an embodiment, the fluid compositionincludes at least CO₂. In some embodiments, the fluid compositionincludes H₂O and optionally CO₂. For example, in an embodiment, thefluid composition includes at least H₂O. In some embodiments, the fluidcomposition includes CO₂ and H₂O. In some embodiments, the fluidcomposition may be air (e.g., for detecting a presence of SO₂ at certainlevels, CO₂ at certain levels, and/or water at certain levels).

The concentration of SO₂ in the fluid composition may range from greaterthan about 0 wt % to about 99.9 wt %. In many embodiments, theconcentration of SO₂ in the fluid composition is less than about 7 wt %.In preferred embodiments, the concentration of SO₂ in the fluidcomposition is less than about 500 ppm. In other preferred embodiments,the concentration of SO₂ in the fluid composition is about 25 ppm orgreater. In other preferred embodiments, the concentration of SO₂ in thefluid composition may range from about 25 ppm to about 500 ppm. In someembodiments, the concentration of CO₂ in the fluid composition is in therange of about 400 ppm to about 5000 ppm. In some embodiments, theconcentration of H₂O in the fluid composition is equivalent to a fluidcomposition having a relative humidity in the range of about 0.01% RH toabout 100% RH.

The step 102 includes sorbing one or more of SO₂, CO₂, and H₂O from thefluid composition on the metal-organic framework. The sorbing mayinclude one or more of adsorbing, absorbing, and desorbing. In anembodiment, the sorbing may include absorbing and/or adsorbing one ormore of SO₂, CO₂, and H₂O. In an embodiment, the sorbing may includeabsorbing one or more of SO₂, CO₂, and H₂O. In an embodiment, thesorbing may include adsorbing one or more of SO₂, CO₂, and H₂O. In anembodiment, the sorbing may include absorbing one or more of SO₂, CO₂,and H₂O. In an embodiment, the sorbing may include desorbing one or moreof SO₂, CO₂, and H₂O. The sorbing may include one or more of selectivesorption (e.g., sorption of one or more select compounds), sequentialsorption (e.g., sorption in a sequence of species and/or sorption inwhich a sorbed species is replaced by another species), and simultaneoussorption (e.g., sorption of two or more compounds, such as two or moreselect compounds). The sorbing may proceed under conditions that are thesame as or similar to the conditions of the contacting.

In some embodiments, the sorbing includes sorbing SO₂. In someembodiments, the sorbing includes sorbing SO₂ over CO₂. In someembodiments, the sorbing includes sorbing SO₂ and CO₂ aboutsimultaneously or sequentially. In some embodiments, the sorbingincludes sorbing SO₂ in the presence of H₂O. In some embodiments, thesorbing includes sorbing SO₂ over CO₂ in the presence of H₂O. In someembodiments, the sorbing includes sorbing SO₂ and CO₂ aboutsimultaneously or sequentially in the presence of H₂O. In someembodiments, the sorbing includes sorbing CO₂. In some embodiments, thesorbing includes sorbing H₂O. In some embodiments, the sorbing includessorbing CO₂ over H₂O. In some embodiments, the sorbing includes sorbingCO₂ in the presence of H₂O. In some embodiments, the sorbing includessorbing H₂O in the presence of CO₂. In some embodiments, the sorbingincludes sorbing CO₂ and H₂O about simultaneously or sequentially.

In some embodiments, the fluid composition includes SO₂ at aconcentration in the range of about 25 ppm to about 500 ppm. In someembodiments, the fluid composition includes SO₂ and CO₂, and SO₂ ispreferentially sorbed over CO₂ on the metal-organic framework. In someembodiments, the fluid composition includes SO₂ and CO₂, and SO₂ and CO₂are both sorbed on the metal-organic framework. In some embodiments, SO₂and CO₂ are sorbed about simultaneously on the metal-organic framework.In some embodiments, the fluid composition includes CO₂ at aconcentration in the range of about 400 ppm to about 5000 ppm. In someembodiments, the fluid composition includes CO₂ and H₂O, and CO₂ ispreferentially sorbed over H₂O on the metal-organic framework. In someembodiments, the fluid composition includes CO₂ and H₂O, and CO₂ and H₂Oare both sorbed on the metal-organic framework. In some embodiments, theCO₂ and H₂O are sorbed about simultaneously on the metal-organicframework. In some embodiments, the sorbing proceeds at about roomtemperature.

In some embodiments, the metal-organic frameworks may exhibit one ormore of a high removal efficiency and/or high uptake, even at lowconcentrations of SO₂. For example, the metal-organic frameworks mayexhibit a removal efficiency of greater than about 70%, greater thanabout 80%, and/or greater than about 90%. In many embodiments, themetal-organic frameworks exhibit a removal efficiency of greater thanabout 90%. For example, the metal-organic frameworks may exhibit aremoval efficiency of greater than about 91%, greater than about 92%,greater than about 93%, greater than about 94%, greater than about 95%,greater than about 96%, greater than about 97%, greater than about 98%,and/or greater than about 99%. The metal-organic frameworks may exhibita high uptake of SO₂ even at low concentrations, such as concentrationsof SO₂ ranging from 25 ppm to 500 ppm.

The metal-organic frameworks may exhibit an about equal selectivitytoward SO₂ and CO₂ and/or a selectivity toward SO₂ over CO₂. In anembodiment, the metal-organic framework includes NbOFFIVE-1-Ni. TheNbOFFIVE-1-Ni may exhibit equal (e.g., about equal) selectivity towardSO₂ and CO₂. In these embodiments, the NbOFFIVE-1-Ni may exhibitsimultaneous (e.g., about simultaneous and/or substantiallysimultaneous) sorption of SO₂ and CO₂, even at low concentrations of SO₂and/or in a presence of water (e.g., water vapor, humidity). Forexample, the NbOFFIVE-1-Ni may simultaneously sorb SO₂ and CO₂, where aconcentration of SO₂ is less than about 500 ppm. In another embodiment,NbOFFIVE-1-Ni may exhibit a selectivity toward SO₂ over CO₂.

In an embodiment, the metal-organic framework includes AlFFIVE-1-Ni. TheAlFFIVE-1-Ni may exhibit a reduced affinity for CO₂ (e.g., relative toNbOFFIVE-1-Ni) such that the AlFFIVE-1-Ni selectively sorbs SO₂ overCO₂, even at low concentrations of SO₂ (e.g., less than about 500 ppm)and/or in a presence of water (e.g., water vapor, humidity). Forexample, a selectivity of SO₂/CO₂ may be about 66. In these embodiments,the AlFFIVE-1-Ni may exhibit a selectivity towards SO₂ over CO₂. In anembodiment, the AlFFIVE-1-Ni may sorb SO₂ to the substantial exclusionof CO₂. In an embodiment, the AlFFIVE-1-Ni may, at first, simultaneouslysorb SO₂ and CO₂ and, over time, SO₂ may replace the sorbed CO₂,demonstrating an overall affinity for SO₂. In another embodiment, theAlFFIVE-1-Ni may exhibit an about equal selectivity for SO₂ and CO₂.

The step 103 is optional and includes regenerating the metal-organicframework. The regenerating may include thermal treatment in a vacuumand/or inert gas environment (e.g., under nitrogen). For example, in anembodiment, the regenerating may include heating to or at a temperatureof about 105° C. in a vacuum. In an embodiment, the regenerating mayinclude heating to or at a temperature of about 105° C. in an inert gasenvironment. In other embodiments, the temperature of regenerating maybe less than and/or greater than about 105° C.

FIG. 2 is a flowchart of a method of a method of capturing chemicalspecies using NiNbOF₅(pyrazine)₂·x(solv), according to one or moreembodiments of the present disclosure. As shown in FIG. 2 , the method200 may comprise contacting 201 a metal-organic framework with a fluidcomposition including one or more of SO₂, CO₂, and H₂O, wherein themetal-organic framework is characterized by the chemical formulaNiNbOF₅(pyrazine)₂·x(solv); sorbing 202 one or more of SO₂, CO₂, and H₂Oon the metal-organic framework; and optionally regenerating 203 themetal-organic framework. In an embodiment, one or more of SO₂, CO₂, andH₂O are sorbed simultaneously (e.g., about simultaneously, substantiallysimultaneously, simultaneously, etc.) on the metal-organic framework.

FIG. 3 is a flowchart of a method of capturing chemical species usingNiAlF₅(H₂O)(pyrazine)₂·x(solv), according to one or more embodiments ofthe present disclosure. As shown in FIG. 3 , the method 300 may comprisecontacting 301 a metal-organic framework with a fluid compositionincluding one or more of SO₂, CO₂, and H₂O, wherein the metal-organicframework is characterized by the chemical formulaNiAlF₅(H₂O)(pyrazine)₂·x(solv); sorbing 302 one or more of SO₂, CO₂, andH₂O on the metal-organic framework; and optionally regenerating 303 themetal-organic framework. In an embodiment, the metal-organic frameworkexhibits a selectivity towards SO₂ over CO₂.

FIG. 4 is a flowchart of a method of sensing, according to one or moreembodiments of the present disclosure. As shown in FIG. 4 , the method400 may comprise exposing 401 a sensor to an environment containing oneor more of SO₂, CO₂, and H₂O; detecting 402 a presence of one or more ofSO₂, CO₂, H₂O in the environment using the sensor; and optionallyregenerating 403 the sensor.

The step 401 includes exposing a sensor to an environment containing oneor more of SO₂, CO₂, and H₂O. The exposing may include subjecting toconditions of an environment. For example, the exposing may includesubjecting the sensor to conditions and/or objects of an environment,which may include, but are not limited to, one or more of temperatureand chemical species present in the environment. The environment may bean environment contaminated or potentially contaminated with SO₂ and/orwith harmful or unsafe levels of CO₂ and/or with harmful or unsafelevels of humidity. The environment may be a dry and/or humidenvironment. For example, in an embodiment, the environment may notinclude any water vapor or negligible amounts of water vapor. In anembodiment, the environment may include non-negligible amounts of watervapor. The environment may be characterized by a relative humidity (RH)ranging from about 0% RH to about 100% RH. For example, in anembodiment, the environment may be characterized by a RH greater thanabout 60% and/or less than about 40%. The environment may becharacterized by any temperature ranging from about 0° C. to about 600°C. In many embodiments, the temperature of the environment may be lessthan about 200° C. In preferred embodiments, the temperature of theenvironment may be about 25° C. (e.g., about room temperature).

The metal-organic framework may be deposited as a sensing layer on asubstrate to form a sensor. For example, in many embodiments, the sensorincludes a layer of a metal-organic framework as the sensing layer. Anyof the metal-organic frameworks of the present disclosure may be used asthe sensing layer of the sensor. For example, in an embodiment, thesensor includes a layer of a metal-organic framework as the sensinglayer, wherein the metal-organic framework is NbOFFIVE-1-Ni or ametal-organic framework characterized by the formulaNiNbOF₅(pyrazine)₂·x(solv). In an embodiment, the sensor includes alayer of a metal-organic framework as the sensing layer, wherein themetal-organic framework is AlFFIVE-1-Ni or a metal-organic frameworkcharacterized by the formula NiAlF₅(H₂O)(pyrazine)₂·x(solv). Themetal-organic frameworks may be uniformly deposited (e.g., aboutuniformly deposited) on a substrate with low intergranular voids and/orrandom orientation. The layer may include crystallites ranging in sizefrom about 150 nm to about 30 μm. The substrate may include any suitablesupport and/or substrate known in the art and/or commonly used insensors. In a preferred embodiment, the substrate includes a quartzcrystal microbalance (QCM) substrate. In another preferred embodiment,the substrate includes a capacitive interdigitated electrode (IDE)substrates. The sensors may further comprise any additional componentsknown in the art and/or commonly included in sensors.

The step 402 includes detecting a presence of one or more of SO₂, CO₂,and H₂O in the environment using the sensor. The detecting may includemeasuring and/or monitoring a change in an electronic or physicalproperty of the sensor in response to an interaction between the sensorand one or more chemical species, such as one or more of SO₂, CO₂, andH₂O. The detecting may be used to determine a presence and/orconcentration of one or more chemical species. In some embodiments, theinteraction may be characterized as a change in an electronic orphysical property of the sensor upon sorbing and/or desorbing one ormore chemical species. The sorbing and/or desorbing may proceed asdescribed herein. A change in capacitance may be measured in response tothe sorption and/or desorption of one or more chemical species. A changein resonance frequency may be measured in response to the sorptionand/or desorption of one or more chemical species. A change inelectrical resistance may be measured in response to the sorption and/ordesorption of one or more chemical species. The electronic propertiesthat may be monitored and/or measured include, but are not limited to,one or more of resonance frequency, capacitance, resistance,conductance, and impedance, among others.

In some embodiments, the detecting proceeds at about room temperature.In some embodiments, the detecting includes detecting SO₂ optionally inthe presence of H₂O. In some embodiments, the detecting includesdetecting between 25 ppm SO₂ to about 500 ppm SO₂ in the environment. Insome embodiments, the detecting includes detecting CO₂ optionally in thepresence of H₂O. In some embodiments, the detecting includes detectingbetween about 400 ppm of CO₂ and 5000 ppm of CO₂ in the environment. Insome embodiments, the detecting includes detecting H₂O optionally in thepresence of CO₂. In some embodiments, the detecting includes detectingrelative humidity levels in the environment below about 40% RH and/orgreater than about 60% RH. In some embodiments, the sensor is acapacitive sensor comprising an interdigitated electrode, wherein thesensing layer is deposited on the interdigitated electrode of thecapacitive sensor, wherein the presence of one or more of SO₂, CO₂, andH₂O is detected by measuring a change in capacitance in the sensinglayer. In some embodiments, the sensor is a QCM sensor comprising anelectrode, wherein the sensing layer is deposited on the electrode ofthe QCM, wherein the presence of one or more of SO₂, CO₂, and H₂O isdetected by measuring a change in resonance frequency in the sensinglayer.

The step 403 is optional and includes regenerating the sensor. Theregenerating may include thermal treatment in a vacuum and/or inert gasenvironment (e.g., under nitrogen). For example, in an embodiment, theregenerating may include heating to or at a temperature of about 105° C.in a vacuum. In an embodiment, the regenerating may include heating toor at a temperature of about 105° C. in an inert gas environment. Inother embodiments, the temperature of regenerating may be less thanand/or greater than about 105° C.

FIG. 5 is a flowchart of a method of sensing using a sensor based onNbOFFIVE-1-Ni, according to one or more embodiments of the presentdisclosure. As shown in FIG. 5 , the method may comprise exposing 501 asensor to an environment containing one or more of SO₂, CO₂, and H₂O,wherein the sensor includes a layer of a metal-organic framework as asensing layer; wherein the metal-organic framework is characterized byNiNbOF₅(pyrazine)₂·x(solv); detecting 502 a presence of one or more ofSO₂, CO₂, and H₂O in the environment using the sensor; and optionallyregenerating 503 the sensor.

FIG. 6 is a flowchart of a method of sensing using a sensor based onAlFFIVE-1-Ni, according to one or more embodiments of the presentdisclosure. As shown in FIG. 6 , the method 600 may comprise exposing601 a sensor to an environment containing one or more of SO₂, CO₂, andH₂O, wherein the sensor includes a layer of a metal-organic framework asa sensing layer; wherein the metal-organic framework is characterized byNiAlF₅(H₂O)(pyrazine)₂·x(solv); detecting 602 a presence of one or moreof SO₂, CO₂, and H₂O in the environment using the sensor; and optionallyregenerating 603 the sensor.

The following Examples are intended to illustrate the above inventionand should not be construed as to narrow its scope. One skilled in theart will readily recognize that the Examiners suggest many other ways inwhich the invention could be practiced. It should be understand thatnumerous variations and modifications may be made while remaining withinthe scope of the invention.

Example 1 Fluorinated MOF Platform to Address the Highly ChallengingSelective Removal and Sensing of SO₂ from Flue Gas and Air

The present Example relates to the use of isostructural fluorinated MOFsfor (i) selective removal of SO₂ from synthetic flue gas and (ii)sensing of SO₂ using QCM as a transducer since the coating of MOFs onthe QCM electrodes can detect the change in mass of sub nanograms uponadsorption or desorption of molecules by the MOF layer. The presentExample describes an unprecedented concurrent removal of SO₂/CO₂ fromsynthetic flue gas and remarkable detection capability in ppm level ofSO₂ concentration in both dry and humid conditions.

Conventional SO₂ scrubber agents, namely calcium oxide and zeolites, areoften used to remove SO₂ utilizing a strong/irreversibleadsorption-based process. However, adsorbents capable of sensing andselectively capturing this toxic molecule with reversibility have yet tobe explored. The present Example describes novel selective removal andsensing of SO₂ using fluorinated metal-organic frameworks (MOFs).Single/mixed gas adsorption experiments were performed at lowconcentrations ranging from about 100 ppm to about 7% of SO₂. Directmixed column breakthrough and/or indirect mixed column breakthroughdesorption experiments revealed an unprecedented SO₂ affinity forNbOFFIVE-1-Ni and AlFFIVE-1-Ni MOFs. Furthermore, MOF-coated quartzcrystal microbalance (QCM) transducers were used to develop sensors withthe ability to detect SO₂ at low concentrations ranging from about 25 toabout 500 ppm.

Methods and Procedures Column Breakthrough Test Set-up, Procedure, andMeasurements

The experimental set-up used for dynamic breakthrough measurements isshown in FIG. 7 . The gas manifold consisted of three lines fitted withmass flow controllers. Line “A” was used to feed an inert gas, mostcommonly helium, to activate the sample before each experiment. Theother two lines, “B” and “C” fed pure or pre-mixed gases. Wheneverrequired, gases flowing through lines “B” and “C” were mixed beforeentering a column packed with the sample using a four-way valve. In atypical experiment, about 300-500 mg of adsorbent (in the column) wastreated in situ at required temperature under He flow (about 50 cm³/g)for about 8 hours.

Before starting each experiment, helium reference gas was flushedthrough the column and then the gas flow was switched to the desired gasmixture at the same flow rate between about 10-40 cm³/g. The gas mixturedownstream the column was monitored using a Hiden mass-spectrometer.

After water saturation (as detected by mass spectrometer), humid He flowwas allowed to continue for two more hours. At this point gas flow waschanged to about 500 ppm SO₂ with balance N₂ (dry, about 23 cc/min flowrate) for about two hours. Adsorbed phase was analyzed by TPD experimentby increasing the temperature of the column under He flow (about 15cc/min). The TPD experiment results showed that in case ofNbOFFIVE-1-Ni, SO₂ was able to replace adsorbed water relatively moreeasily than AlFFIVE-1-Ni. The results were on expected line consideringrelative water affinity of both the compound and further support trendin sensing experiments.

Fabrication of NbOFFIVE-1-Ni and AlFFIVE-1-Ni Coated QCM:

The transducer was a 10 MHz AT-cut piezoelectric quartz crystal quartzmicrobalance (QCM) device with a thickness shear mode and placed betweentwo gold electrodes for electrical connection. The QCM was rinsed withethanol and dried in air. MOFs paste was then applied to the electrodeof QCMs by spin-coating method (2 μm thick) no prior modification of thesensors surface was required.

The QCM sensor was then fixed in a sealed chamber. Prior to measurementsthe fresh coated MOFs film was activated in situ for about 4 hours tohave a guest free framework. The resulting coatings were ultrathin andreproducible so that the stress upon absorption of SO₂ inducing a changein the mass change of the thin film was effective.

Apparatus:

FIG. 8 shows the sensing set-up used in this Example for real-timemeasurement. All the sensor measurements were carried out at about roomtemperature, under a dry air total stream of about 200 sccm. MFCs (Massflow controllers) from Alicat scientific Inc. were used to control theflow rate for gases coming from certified bottles. Stainless steeldelivery lines or perfluoroalkoxy alkane, PFA tubing (in regionsrequiring flexibility and resistivity to VOCs) were used on the setupwith Vernier metering valves (from Swagelok) as a flow regulator. Todetect the change in humidity level inside the chamber a commercialhumidity sensor (Honeywell HIH-4000-003) was used as a reference whichhas an error less than about 0.5% RH. The QCM sensor was exposed to theanalyte stream until a stable response was obtained, a two-port network(Keysight E5071C ENA) circuit was used to monitor the change inresonance frequency. A LabVIEW interface was used for synchronizationand data acquisition by controlling the LCR meter and the multimeter.Hence, the possibility of data loss was minimized.

Results and Discussion

The fluorinated MOF platforms, namely NbOFFIVE-1-Ni and AlFFIVE-1-Ni,resulted in many desirable properties. Although both of the MOFs areisostructural, the subtle differences in their chemical compositions,(NbOF₅)²⁻ instead of AlF₅(H₂O)²⁻, allowed the modulation of theirproperties by varying the content and intermolecular spacing of pendingfluoride groups realized via different tilts of pyrazine molecules(FIGS. 9A-9C). In view of the excellent stability and the modular natureof these MOF materials, their use for SO₂ removal and sensing insynthetic flue gas and air, respectively, was investigated.

SO₂ Removal from Flue Gas

NbOFFIVE-1-Ni was first investigated for SO₂ sorption. The steep, pureSO₂ adsorption isotherm collected at about 25° C. (FIG. 10 ) suggested ahigh affinity of the NbOFFIVE-1-Ni framework for SO₂. This observationwas corroborated by Density Functional Theory (DFT) calculations, whichrevealed high SO₂/NbOFFIVE-1-Ni interaction energy of about −64.8kJ/mol. This was due to a relatively stronger interaction between thesulfur atom of SO₂ and the F-pillars with characteristic interatomicdistances of about 2.9 Å (FIG. 11A) along with a charge transfer betweenthe guest and this region of the MOF. Interestingly, theSO₂/NbOFFIVE-1-Ni interaction energy was similar to the value calculatedfor CO₂ (about −54.5 kJ/mol). This latter molecule occupied slightlydifferent sites than SO₂, implying an interaction of the guest moleculewith both the F-pillars and the pyrazine groups (FIG. 11B). Theso-predicted energetics for a single gas behavior suggested simultaneouscapture of SO₂ and CO₂.

Cyclic adsorption column breakthrough tests with SO₂/N₂:7/93 indicatestability and good uptake (≈2.2 mmol/g) of SO₂ (FIGS. 12A-12D).Furthermore, adsorption column breakthrough experiments with SO₂/CO₂/N₂:4/4/92 gas mixture showed simultaneous and equal retention time in thecolumn for SO₂ and CO₂, demonstrating identical uptake of ≈1.1 mmol/g(FIG. 13 ), which was consistent with the simulated energetics trends.Upon decreasing the SO₂ concentration with nitrogen in the rangecommonly observed in flue gas (500 ppm) (SO₂/N₂: 0.05/99.95 mixture),NbOFFIVE-1-Ni maintained a high SO₂ uptake of about 1.4 mmol/g.Interestingly, adsorption column breakthrough experiments under mimickedflue gas conditions with about 500 ppm of SO₂ and about 10% CO₂ in N₂(SO₂/CO₂/N₂: 0.05/10/89.95) resulted in equal and simultaneous retentiontime for both SO₂ and CO₂, leading to uptakes of ≈0.01 mmol/g and ≈2.2mmol/g, respectively. This direct co-adsorption experiment demonstratedthat NbOFFIVE-1-Ni exhibited equal selectivity toward SO₂ and CO₂, whichis desirable for simultaneous CO₂ and SO₂ capture in flue gas(containing low SO₂ concentrations). Nevertheless,temperature-programmed desorption (TPD) confirmed the presence of CO₂only with an undetectable amount of SO₂ (FIG. 12D) in the adsorbed phaseas the amount of SO₂ adsorbed was negligible owing to its lowconcentration.

In the quest for a material with more favorable selectivity for SO₂removal from flue gas than CO₂ (at 500 ppm of SO₂), an analogue ofNbOFFIVE-1-Ni with lower CO₂ interactions and potentially higher SO₂interactions was investigated. In particular, AlFFIVE-1-Ni was exploredfor the structural SO₂/CO₂ co-adsorption property. AlFFIVE-1-Niexhibited three pendant fluoride groups with slightly higher F . . . Fdistance (3.613 Å) and one potential open metal site, whereasNbOFFIVE-1-Ni contained four pendants fluoride with smaller F . . . Fdistance (3.210(8) Å) and no open metal site. Such minute differences instructural features led to a discovery of equal selectivity for CO₂ andH₂S over a wide range of concentrations and temperatures. Encouraged bythis structure-property tuning of H₂S and CO₂ adsorption affinity usingthis MOF, AlFFIVE-1-Ni was expected to be more selective toward SO₂ thanCO₂. The DFT calculations first revealed a lowering of the host/guestinteraction energy of CO₂ for AlFFIVE-1-Ni compared to NbOFFIVE-1-Ni(−47.0 kJ/mol vs. −54.5 kJ/mol). In the case of AlFFIVE-1-Ni, thetrigonal bipyramidal-like Al³⁺ environment did not allow for furtheroptimal interactions between a carbon atom in CO₂ and four F-pillars(FIG. 11D), as seen in NbOFFIVE-1-Ni. Interestingly, the simulatedpreferential location of SO₂ was slightly pushed toward the pore wall,as compared to the scenario in NbOFFIVE-1-Ni, with the formation of adual interaction between its sulfur atoms and the two nearby F-pillarsas well as its oxygen atoms interacting with the pyrazine linker withshorter interacting distances (FIG. 11C). The resulting geometry led toa slight enhancement of the SO₂/host interaction energy (−67.3 kJ/mol)and reduced affinity toward CO₂, making AlFFIVE-1-Ni a promisingcandidate to selectively adsorb SO₂ over CO₂.

Investigation of single SO₂ adsorption showed that AlFFIVE-1-Ni alsoexhibited a steep adsorption isotherm at about 25° C. (FIG. 14 ). Thecorresponding adsorption column breakthrough experiment with SO₂/N₂:7/93 mixture showed a high uptake of about 2.2 mmol/g (FIGS. 15A-15D).AlFFIVE-1-Ni can be completely regenerated by heating at about 105° C.in a vacuum or inert gas environment (FIG. 16 ). During the adsorptioncolumn breakthrough experiments carried out with low SO₂ (SO₂/N₂:0.05/99.95) mixture, AlFFIVE-1-Ni still maintained a high uptake of SO₂(about 1.6 mmol/g). Subsequent TPD analysis of the adsorbed phaseconfirmed the adsorption of SO₂ (FIG. 17 ) at ppm level. Adsorptioncolumn breakthrough experiments with synthetic flue gas using aSO₂/CO₂/N₂: 0.05/10/89.95 mixture showed that SO₂ continues to beadsorbed for long durations past the CO₂ breakthrough. This indicatedthat the adsorbed CO₂ was replaced by SO₂ from the gas mixture, whichwas consistent with a much higher estimated interaction energy of SO₂over CO₂. Subsequent TPD analysis suggested an adsorbed phasecomposition of about 1.5 mmol/g for CO₂ and about 0.5 mmol/g for SO₂,which was remarkable considering the large difference in concentrationsof CO₂ and SO₂ in the synthetic flue gas (FIG. 15D). A selectivity ofSO₂/CO₂≈66 showed that AlFFIVE-1-Ni was one of the most efficientmaterials for SO₂ removal at a ppm level and is promising forselectively removing SO₂ from flue gas.

Selective SO₂ Detection from the Air

From the adsorptive separation study above, AlFFIVE-1-Ni andNbOFFIVE-1-Ni were shown to exhibit tunable CO₂/H₂S selectivity,molecules that are present in environments contaminated with SO₂. Tobenefit from the outstanding properties of this platform, thefeasibility of depositing AlFFIVE-1-Ni and NbOFFIVE-1-Ni on a QCMelectrode and unveiling their SO₂ sensing properties in the presence andabsence of humidity to mimic atmospheric conditions were explored.

The surface morphology of AlFFIVE-1-Ni and NbOFFIVE-1-Ni coated on QCM(see inset) was studied using scanning electron microscopy (SEM). Thethin films of both MOFs were found to be compact and uniform. Thedensely packed MOFs crystals were uniformly deposited on the QCMsubstrate with low intergranular voids and random orientation. Asillustrated in FIGS. 18A-18B, the coating of NbOFFIVE-1-Ni led to cubiccrystallites of approximately 150 nm, while for the AlFFIVE-1-Ni films,the size of the crystallites was significantly larger at ˜30 μm. PowderX-ray diffraction experiments were carried out to confirm the purity andcrystallinity of the deposited MOFs (FIGS. 19A-19B).

The sensitivity (Δf/f)) of AlFFIVE-1-Ni and NbOFFIVE-1-Ni coated QCMdevices were measured for different concentrations of SO₂, ranging from0 to 500 ppm in nitrogen. Uncoated QCM showed a negligible response toSO₂. With the increase in the concentration of SO₂, both MOF-coatedsensors responded with a nonlinear decrease in sensitivity (FIG. 20 )and (FIGS. 21A-21B). After each exposure cycle, the device was in situheated at about 105° C. in ambient nitrogen for about four hours, whichreactivated the MOF thin films for sensing.

Humidity is present in most environments, and so it was important tounderstand a sensor's response in its presence. Therefore, mixed gasexperiments were performed, exposing NbOFFIVE-1-Ni and AlFFIVE-1-Ni toSO₂ in humid conditions mimicking real-world conditions. FIG. 22 showsthe sensor sensitivity as a function of SO₂ concentration in humidconditions (60% RH) at room temperature for uncoated and coatedNbOFFIVE-1-Ni, AlFFIVE-1-Ni QCMs. Uncoated QCM had a near zero responseto humidity and SO₂. This corroborated that the sensing response to SO₂under humid conditions was due to its affinity to NbOFFIVE-1-Ni andAlFFIVE-1-Ni films.

The responses of two kinds of sensors were different. As seen in FIG. 22and (FIGS. 23A-23B), the resonance frequency of the QCMs initiallydecreased when the ambience was changed from dry to humid SO₂conditions. The most prominent difference was the inversion in thesensor output due to the introduction of SO₂ at 60% RH, but not in thesame manner as compared to the dry SO₂ case. Interestingly, when exposedto 25 ppm of SO₂ in the above-mentioned humid conditions, the sensorresonance frequency for SO₂ was reduced. Under humid conditions, thesensitivity of the two MOFs was slightly reduced when compared to dryconditions. However, NbOFFIVE-1-Ni films demonstrated a four-time highersensitivity toward SO₂ in the presence of humidity compared toAlFFIVE-1-Ni.

To further analyze the results obtained, it was necessary to considerthe specific features of the adsorption of SO₂ and water on the surfaceof NbOFFIVE-1-Ni and AlFFIVE-1-Ni. As seen in FIG. 22 , the presence ofhumidity (60% RH) did not significantly affect the NbOFFIVE-1-Ni basedsensor's response to the SO₂ analyte. This may be due to the affinity ofSO₂ molecules to replace some of the adsorbed water molecules or/andcoexist in the highly confined pores. In the case of AlFFIVE-1-Ni basedsensor, which was isomorphic to the NbOFFIVE-1-Ni, lower sensitivity toSO₂ in the presence of humidity was observed. Although SO₂ has theaffinity to replace water molecules, the reduced sensitivity wasattributed to the absence of accessible ultra-microporous morphology.The number of SO₂ adsorbing active sites was reduced by the pre-adsorbedwater, thereby limiting the available space for adsorption. Thisobservation was supported by the fact that the water molecules stronglyinteracted with Al³⁺ with higher host/guest interaction energy ascompared to SO₂. The TPD experiment results (FIG. 24 ) showed that inthe case of NbOFFIVE-Ni-1, the adsorbed SO₂ was replaced relativelyeasily with water molecules as compared to the co-adsorption of SO₂ andH₂O in AlFFIVE-1-Ni or coexisted with the adsorbed water in the confinedpores of NbOFFIVE-Ni-1.

The most important parameters of a sensing device are its stability andreproducibility. These parameters were investigated by cyclic exposureof the sensor to different SO₂ concentrations after every forty-eighthours at about room temperature over a period of about twelve days (FIG.25A-25B). The three results demonstrated the stability of the sensorsexposed to about 50, 100, and 157 ppm SO₂ gas with no significant changein the resonant frequency over time.

In summary, the superior performance of two fluorinated MOFs, namelyNbOFFIVE-1-Ni and AlFFIVE-1-Ni, for the capture of SO₂ from flue gas wassuccessfully demonstrated. Combined single/mixed gas breakthroughexperiments and molecular simulation confirmed that simultaneous captureof SO₂ and CO₂ occurred using NbOFFIVE-1-Ni, while AlFFIVE-1-Nidisplayed a higher affinity for SO₂ with SO₂/CO₂ selectivity ≈66. Basedon this performance, QCM-based sensors were successfully fabricated forsensing SO₂ from air using this fluorinated MOF platform (FIG. 26 ).Both MOF materials confirmed their potential, revealing good SO₂detection capabilities above 25 ppm, the range of SO₂ concentrations inthe air-inducing nose and eye irritation. This remarkable performance ofsensing made these materials highly desirable for the fabrication of newadvanced devices to improve health and environmental conditions.

Example 2 Concurrent Sensing of CO₂ and H₂O from Air UsingUltramicroporous Fluorinated Meta-Organic Frameworks

Conventional materials for gas/vapor sensing are limited to a singleprobe detection ability for specific analytes. However, materialscapable of concurrent detection of two different probes in theirrespective harmful levels and using two types of sensing modes have yetto be explored. In particular, the concurrent detection of uncomfortablehumidity levels and CO₂ concentration (400-5000 ppm) in confined spacesis of extreme importance in a great variety of fields, such as submarinetechnology, aerospace, mining, and rescue operations. The followingExample reports the deliberate construction and performance assessmentof extremely sensitive sensors using an interdigitated electrode(IDE)-based capacitor and a quartz crystal microbalance (QCM) astransducing substrates. The unveiled sensors were able to simultaneouslydetect CO₂ within the 400-5000 ppm range and relative humidity levelsbelow 40 and above 60%, using two fluorinated metal-organicframeworks-namely, NbOFFIVE-1-Ni and AlFFIVE-1-Ni-fabricated as thinfilms. Their subtle difference in a structure-adsorption relationshipfor H₂O and CO₂ was analyzed to unveil the correspondingstructure-sensing property relationships using both QCM- and IDE-basedsensing modes.

Metal-organic frameworks (MOFs) are a unique class of porous materialsthat have shown great potential for gas separation/storage, catalysis,and sensing. Recently, the use of a fluorinated MOF, namely,AlFFIVE-1-Ni, as adsorbent has offered the ability to simultaneouslyremove H₂O and CO₂ from various gas streams. This property isdistinctive and remarkable as the capture of both gases usually followsa competitive adsorptive mechanism. In fact, the presence of twodistinct actives sites, those on open metal sites for the adsorption ofH₂O and those within cavities for the adsorption of CO₂, explains thisunique simultaneous adsorption process of the AlFFIVE-1-Ni adsorbent.

In particular, this Example presents the development of the firstsensing device with the ability to simultaneously detect and measure CO₂and H₂O. In this study, harmful levels of CO₂ (between 400 and 5000 ppm)and uncomfortable levels of humidity (below 40% RH and higher than 60%RH) commonly present in indoor environments were established as targetsfor ranges of detection. Gas sensitivity performances were analyzedusing two different transduction techniques: one measuring changes inmass (using QCM) and the other measuring changes in dielectricproperties (using an interdigitated electrode (IDE) capacitor). Theperformance of AlFFIVE-1-Ni was compared with those obtained usinganother fluorinated MOF, NbOFFIVE-1-Ni, which displayed a competitiveadsorption process of CO₂ and H₂O.

Materials and Methods Materials

All solvents and reagents were used without further purification:Ni(NO₃)₂·6H₂O (Acros), Al(NO₃)₃·9H₂O (Aldrich), pyrazine (Aldrich),Nb₂O₅ (Aldrich), Ni(NO₃)₂·6H₂O (Acros), and HF (Aldrich).

AlFFIVE-1-Ni

Pyrazine (384.40 mg, 4.80 mmol), Ni(NO₃)₂·6H₂O (174.50 mg, 0.60 mmol),Al(NO₃)₃·9H₂O (225.0 mg, 0.6 mmol), and HF (aqueous, 48%, 0.26 ml, 7.15mmol) were mixed in a 20 Ml Teflon-lined autoclave. After dilution ofthe mixture with 3 mL of deionized water, the autoclave was sealed andheated to 85° C. for 24 h. After cooling the reaction mixture to roomtemperature, the obtained blue-violet square-shaped crystals, suitablefor single-crystal X-ray structure determination, were collected byfiltration, washed with ethanol, and dried in air. Elemental analysis: N%, 13.76 (theor: 14.19), C %, 21.73 (theor: 24.33), H %, 3.16 (theor:3.57). NiAlF₅(H₂O)(pyr)₂·2H₂O (called AlFFIVE-1-Ni) was activated at105° C. for one night under high vacuum (3 mTorr) before each adsorptionmeasurements.

NbOFFIVE-1-Ni

Pyrazine (384.40 mg, 4.80 mmol), Ni(NO₃)₂·6H₂O (174.50 mg, 0.60 mmol),Nb₂O₅ (79.70 mg, 0.30 mmol), and HF (aqueous, 48%, 0.26 mL, 7.15 mmol)were mixed in a 20 mL Teflon-lined autoclave. The mixture was dilutedwith 3 mL of deionized water, and the autoclave was then sealed andheated to 130° C. for 24 h. After cooling the reaction mixture to roomtemperature, the obtained violet square-shaped crystals, suitable forsingle-crystal X-ray structure determination, were collected byfiltration, washed with ethanol, and dried in air. Elemental analysisC₈H₁₂O₂N₄F₅NiNb: N %, 11.88 (theor: 12.21), C %, 20.58 (theor: 20.54), H%, 2.54 (theor: 2.64), 0%, 11.42 (theor: 10.46). NiNbOF₅(pyr)₂·(H₂O)₂(called NbOFFIVE-1-Ni) was activated at 105° C. for 12 h under highvacuum (3 mTorr) before each adsorption experiment.

QCM and IDE Electrodes.

IDE-based capacitors were fabricated on a highly resistive silicon waferusing complementary metal oxide semiconductor processes. A 2 μm silicondioxide layer was grown using wet thermal oxidation for electricalisolation. A layer of 10/300 nm Ti/Au was subsequentlysputtered-deposited via physical vapor deposition in a ESC metal sputtersystem. Photolithography was used in the next step of the process, todefine the IDEs (4 μm fingers with 5 μm spaces). The metal layer wasthen etched using an ion sputtering system PlasmaLab System from OxfordInstruments, with the patterned photoresist acting as the mask layer.AT-cut QCM (10 MHz) with 6 mm diameter electrodes from openQCM was usedas substrate for the mass-based sensing technique.

Fabrication of NbOFFIVE-1-Ni- and AlFFIVE-1-Ni-Coated IDE/QCM.

Electrodes were rinsed with acetone/ethanol and dried in air. MOF pastewas then deposited on one of the electrode of QCMs/IDEs by spin coatingmethod (2 μm thick). No prior modification of the sensor surface wasrequired for this method of deposition. The method was simple inyielding good quality and uniform films. The coated electrodes were thendried at 60° C. for 2 h under vacuum to obtain thin films of sensingmaterials on the electrodes. The sensors were then characterized in acustom-built sealed chamber. Before any measurements, the freshly coatedMOF film on the sensors was activated in situ for 4 h to have aguest-free framework. The resulting coatings were ultrathin andreproducible so that the absorbed CO₂ and/or water vapor induced aneffective change in the mass/dielectric properties of the thin films.

Apparatus.

FIG. 8 shows the schematic of the setup used in this study for real-timegas sensing measurements. Mass flow controllers from Alicat Scientific,Inc. were used to control the flow rate of gases from certified bottles.Stainless steel or PFA tubing (in regions requiring flexibility) alongwith Vernier metering valves (from Swagelok) as a flow regulator wasused as delivery line in the setup. A commercial humidity sensor(Honeywell HIH-4000-003, error less than 0.5% RH) was used to monitorthe humidity levels inside the test chamber. The QCM-/IDE-based sensorswere exposed to the analyte stream until a stable response was attained.A two-port impedance analyzer (Keysight E5071C ENA) circuit was used formonitoring the change in resonance frequency. A LabVIEW interface wasused for synchronization and data acquisition by controlling the LCRmeter and the multimeter. This minimized the possibility of data loss.

Results and Discussion Preparation and Characterization of SensingMaterials

Recently, reticular chemistry allowed the fabrication of a series offluorinated MOF materials with the ability to capture CO₂ from air or todehydrate gas streams. The structure of these materials can be describedas having a pcu underlying topology, where a square-grid Ni(pyrazine)₂is pillared by inorganic building blocks, either [NbOF₅]²⁻ or[AlF₅(H₂O)]²⁻, to generate a channel-based MOF with a periodic array offluorine moieties (FIGS. 9A-9C). The structural differences betweenthese two fluorinated MOFs resulted from the presence of an open metalsite in the case of AlFFIVE-1-Ni. Markedly, gas adsorption experimentson AlFFIVE-1-Ni revealed the simultaneous adsorption of CO₂ and H₂Omolecules. In situ single-crystal X-ray diffraction and DFT calculationsshowed that the open metal sites were the preferred adsorption sites forH₂O molecules, whereas CO₂ preferably adsorbed within the cavities.Desorption experiments at set temperatures conducted after adsorption ofa mixture of CO₂ and H₂O confirmed the concomitant nature of theadsorption mechanism. Similar experiments conducted with NbOFFIVE-1-Ni,an analogue with no open metal site, confirmed the competitive nature ofthe adsorption mechanism, with preferential adsorption of CO₂ to onesingle site located within the cavity.

On the basis of these remarkable adsorptive properties, AlFFIVE-1-Ni andNbOFFIVE-1-Ni are expected to be excellent candidates for addressingchallenges faced by many chemical sensors in the concurrent detection ofCO₂ and H₂O. To do so, there is a need for establishing a signaltransduction process that enables the use of NbOFFIVE-1-Ni andAlFFIVE-1-Ni for chemical sensing.

In recent years, QCM and IDE technologies have been proposed as neweffective tools for the rapid detection of gases, volatile organiccompounds, and humidity, because of their simplicity, small size, lowcost, high sensitivity, shorter time of analysis, and suitability forlabel-free measurements. The resonant frequency of QCM substratesdepends on the amount of adsorbed material. QCM can detect the change ina mass of subnanograms. The relationship between the shift in frequencyand the mass loading is described by the Sauerbrey equation. CapacitiveIDEs can be used to sense the change in sensing film permittivity upongas adsorption and are seen as attractive candidates from a powerconsumption perspective.

The key element of any chemical sensor is the sensitive layer thatcaptures the analyte gases. MOF films are generally grown on surfacesthat have been functionalized with self-assembled monolayers or byseeding with small MOF crystal. The nanostructures of these thin filmshave not yet been well characterized and sometimes lead to impropergrowth of the desired thin film. In this context, Applicants havedeveloped, for the first time, a new synthesis method to obtain a softhomogeneous MOF solution with a paste-like consistency, making it wellsuited for the preparation of a wide range of homogeneous thin film.Indeed, the method can easily be adapted to the deposition or spincoating of thin films from a chemical solution. A representation of thisis presented in (FIGS. 27A-27B), which shows a good uniformity in thedeposition of the crystals, along with an improved adhesion to thesubstrates. AlFFIVE-1-Ni and NbOFFIVE-1-Ni films contain close-packedcrystal domains, exhibiting a good coalescence of microcrystals withsmall intergranular voids, leading to compact and uniform MOF films ofexcellent crystallinity. NbOFFIVE-1-Ni films comprised small cubiccrystallites of approximately 150 nm, whereas larger crystallites ofabout 7 μm were found for AlFFIVE-1-Ni. The purity and crystallinity ofthe deposited MOF films were confirmed by powder X-ray diffractionexperiments (FIGS. 28A-28B).

Gas Sensing Properties Concurrent Sensing of CO₂ and H₂O

The use of MOF thin films for sensing application required a criticalactivation step, permitting the attainment of a guest-free thin filmbefore any sensing signal measurement was to be performed. For thisreason, the MOF adsorbent was fully reactivated at 105° C., before eachnew cycle of analyte exposure, using in situ heating provided by a HT24Smetal ceramic heater from Thorlabs. The output temperature of the heaterwas calibrated and monitored using a LM35DZ/NOPB commercial temperaturesensor from Texas Instruments (FIG. 29 ). This calibration was of primeimportance to preserve the integrity of the MOF adsorbent and to ensurethat guest-free activated NbOFFIVE-1-Ni and AlFFIVE-1-Ni were obtainedbefore sensing experiments.

The sensing characteristics and performance of NbOFFIVE-1-Ni andAlFFIVE-1-Ni were investigated at variable CO₂ concentration in dry andhumid conditions to verify the potential application of the newlyfabricated sensors. Interestingly, NbOFFIVE-1-Ni and AlFFIVE-1-Ni coatedon IDE and QCM as sensors exhibited a nonlinear change in the signalover several orders of magnitude of CO₂ concentration (from 400 to 5000ppm). The capacitive properties of the IDE exhibited a dielectricconstant dependency of the material, because of the change in CO₂concentration. The capacitance was calculated using the standardcapacitance equation. The exposure of all sensors to different dry CO₂concentrations led to a sharp decrease in the capacitance (FIGS.30A-30B). Additionally, the interaction of CO₂ with NbOFFIVE-1-Ni andAlFFIVE-1-Ni changed the local dielectric properties of the thin films,resulting in a decrease in capacitance. Remarkably, the interdigitatedsensor devices coated with NbOFFIVE-1-Ni showed a significantly higherchange in capacitance (about 100 times, for 400-5000 ppm of CO₂) whencompared to AlFFIVE-1-Ni. These observations were in agreement with theremarkably favorable selectivity of CO₂ obtained from calorimetric andco-adsorption tests.

FIGS. 30C-30D depict the concentration-dependent responses ofNbOFFIVE-1-Ni or AlFFIVE-1-Ni coated on QCMs as sensors to differentconcentrations of CO₂ at room temperature and under dry conditions. Itwas observed that when the gas entered in contact with the QCM-basedsensors, CO₂ molecules were adsorbed on the sensitive film and the massincreased in a proportional manner, producing a negative shift inresonance frequency. That is, the decrease in resonant frequency (FIGS.30C-30D) as the CO₂ concentration increased was seen very clearly,showing a good response of the QCM to CO₂.

In the case of QCM devices, the maximum shifts in frequency were −84.85Hz for NbOFFIVE-1-Ni and −3.5 Hz for AlFFIVE-1-Ni at CO₂ concentrationsranging from 400 to 5000 ppm. These results were in good agreement withNbOFFIVE-1-Ni exhibiting a higher interaction with CO₂ at a low loading(53 kJ/mol) versus a weaker interaction (45 kJ/mol) for AlFFIVE-1-Ni. Inthe case of IDE sensing mode under dry CO₂ conditions, a high adsorptiveselectivity was observed at low CO₂ loadings and concentrations forNbOFFIVE-1-Ni versus AlFFIVE-1-Ni.

Interestingly, the MOF-coated mass-sensitive QCM devices exhibited alower sensitivity toward CO₂ under dry conditions than the IDEcapacitors. Part of the reason for this difference was that the IDEcapacitors can detect larger effects of CO₂ on dielectric properties ofthe thin films, whereas the mass-sensitive QCM devices were only able todetect small mass changes.

Effect of Synthetic Air on Concurrent Sensing of CO₂ and H₂O.

In general, the moisture from the environment had a considerable impacton the sensitivity of the gas sensor and must be taken intoconsideration for practical deployment of any sensor. Therefore, theconcentration-dependent response of the IDE and QCM devices coated withNbOFFIVE-1-Ni or AlFFIVE-1-Ni to CO₂ under humid conditions wasinvestigated. The relationship between the resonant frequencies of theQCM sensors and the RH (60%) is shown in FIGS. 31A-31D.

The sensitivity of NbOFFIVE-1-Ni coated on QCM as a sensor was reducedin the presence of moisture and was easily distinguishable from theresponse of CO₂ in the air. However, the sensitivity was considerablyreduced with AlFFIVE-1-Ni-coated QCM, and its frequency was reduced evenmore, especially at low CO₂ concentrations. In the case of IDE-typesensing, NbOFFIVE-1-Ni- and AlFFIVE-1-Ni-coated IDE sensors were greatlyaffected by the presence of moisture. In fact, an often heard objectionto capacitive sensor technology is that it is sensitive to humidity, yetthe capacitive sensor is based on dielectric changes of the thin filmupon water vapor uptake. Practically, humidity and condensable vaporsare shown to have a significant effect on the IDE-based sensors becausewater has a dielectric constant εr of 78.3, that is, −48 times largerthan CO₂ (εr=1.60). The presence of moisture often interferes with theIDE sensory signal of CO₂ and thereby can hamper its qualitativeidentification and quantification. Accordingly, the results presentedhere show that QCM-based sensors are a good alternative to compensatethe loss of performance in IDE-based sensors, because of the presence ofhumidity.

A detailed analysis of the sensitivity to H₂O as a function of RH (26 to60% RH) was performed to further delineate the water vapor adsorptivebased sensing performance.

After an initial activation cycle at 105° C., the sensors were cooled toroom temperature and subsequently exposed to different levels of RH,using N₂ as a gas carrier. The RH level was varied by changing thecarrier flow (0 to 200 mL/min) bubbling through the water. It should benoted that an ideal sensor swiftly senses water vapor, in the optimalrange of 26-65% RH. This is the desired range of humidity levels forconfined spaces, conforming to the standards set by the occupationalhealth safety. The humidity sensing behavior of theAlFFIVE-1-Ni-/NbOFFIVE-1-Ni-coated IDE- and QCM-based sensors, as shownin FIGS. 32A-32D, revealed a response in a nonlinear fashion with goodsensitivity in the recommended range of humidity levels (26-65% RH).Notably, AlFFIVE-1-Ni and NbOFFIVE-1-Ni had an almost simultaneousresponse to change in humidity levels; however, as expected, thepresence of an open metal site in highly confined pores, in the case ofAlFFIVE-1-Ni thin films, offered a greater sensitivity to humidity. Infact, the exposed Al³⁺ of the AlFFIVE-1-Ni framework served as theinitial preferable adsorption site for water molecules, when exposed todifferent levels of RH levels.

Humidity levels above 60% have a considerable impact on the sensitivity,which was related to the nature of the MOF adsorbent. AlFFIVE-1-Niexhibited a sensitivity five times higher than NbOFFIVE-1-Ni, in the26-60% RH range. On the other hand, a comparison of the effects of thetransduction mechanisms on the sensing performance under humidconditions clearly showed that QCM-based sensors were more reluctant tochanges than IDE-based devices.

It is to be noted that the shift in frequency and capacitance decreasedmore steeply, when RH exceeded 60%, a phenomenon that can be explainedby the aggregation of water molecules on thin films.

To mimic the real conditions (atmospheric conditions), experiments werealso performed in the presence of 500 ppm CO₂, using both transductiontechniques (FIGS. 33A-33D). After an initial exposure to 500 ppm CO₂,both samples exhibited a decrease in their sensing signals, as afunction of the subsequent exposure to humidity. In the case ofAlFFIVE-1-Ni, this decrease was almost two times higher than thatobserved for NbOFFIVE-1-Ni.

Interestingly, changes in CO₂ levels had a lower impact (four timeslower) for NbOFFIVE-1-Ni than that for AlFFIVE-1-Ni. This suggested thata partial desorption of noncoordinated water molecules occurred easilyfor NbOFFIVE-1-Ni, because of their relatively weak interactionsoccurring with the framework, compared to those of CO₂ with theframework. In the case of AlFFIVE-1-Ni, the water molecules coordinatedto the open metal site could not be desorbed because of their strongerinteraction with the open metal sites in the presence of CO₂, thereforecausing unchanged performances for water detection. Although both QCMand IDE responded to variable H₂O and CO₂ concentrations, thesensitivities were not identical. Accordingly, the detection of CO₂ inthe presence of variable humidity levels under real (atmospheric)conditions were directly observed by a shift in the resonance frequencyof QCM. However, the capacitive sensor did a much better job ofreflecting the changes in humidity levels in the presence of 500 ppmCO₂.

Stability Studies.

The most important parameters for a given sensing device are itsstability and reproducibility. These parameters were investigated bycyclic exposure every 48 h at room temperature for over a period of 12days (FIGS. 34A-34B). It was clearly shown that both sensor responsesdid not vary significantly with time, confirming their long-termstability.

These results pinpointed some of the most important elements requiredfor the choice of materials granting comfortable/healthy indoorconditions; they also shed light on the associated transductionmechanisms required for the concurrent detection of harmful levels ofCO₂ and humidity in a condition akin to atmospheric conditions andconfined environments. The response of the sensors to humidity and CO₂showed that sensitivity and resolution were dependent on both thetransduction mechanism and the sensing material. However, the response,stability, selectivity, detection limit, and life cycle were found todepend only on the intrinsic properties of the sensing material.

In summary, the present Example reports, for the first time, the use ofboth IDE- and QCM-based sensors for the simultaneous detection of CO₂and H₂O using ultramicroporous fluorinated MOFs, exhibitingunprecedented structural CO₂—H₂O adsorption properties. The sensorsexhibited excellent selectivity for sensing CO₂ at variable humiditylevels; they also detected humidity levels at variable CO₂concentrations. It has been shown that isostructural fluorinatedMOF-based capacitance sensitive sensors possessed good CO₂ sensingproperties under dry conditions. When working under real conditions(atmospheric conditions), the change in the dielectric constant over thechange in the CO₂ concentrations and at a constant humidity level of 60%RH resulted in a decrease in capacitance. The detection of CO₂ underreal conditions was directly observed on the QCM frequency shift.However, the capacitive sensor did a better job reflecting the changesin humidity in the presence of CO₂. At room temperature, all thesensor's properties reported herein make the reported sensor a promisingcandidate for the detection of CO₂ and water vapor when used for indoorand confined spaces.

Other embodiments of the present disclosure are possible. Although thedescription above contains much specificity, these should not beconstrued as limiting the scope of the disclosure, but as merelyproviding illustrations of some of the presently preferred embodimentsof this disclosure. It is also contemplated that various combinations orsub-combinations of the specific features and aspects of the embodimentsmay be made and still fall within the scope of this disclosure. Itshould be understood that various features and aspects of the disclosedembodiments can be combined with or substituted for one another in orderto form various embodiments. Thus, it is intended that the scope of atleast some of the present disclosure should not be limited by theparticular disclosed embodiments described above.

Thus the scope of this disclosure should be determined by the appendedclaims and their legal equivalents. Therefore, it will be appreciatedthat the scope of the present disclosure fully encompasses otherembodiments which may become obvious to those skilled in the art, andthat the scope of the present disclosure is accordingly to be limited bynothing other than the appended claims, in which reference to an elementin the singular is not intended to mean “one and only one” unlessexplicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present disclosure, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims.

The foregoing description of various preferred embodiments of thedisclosure have been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit thedisclosure to the precise embodiments, and obviously many modificationsand variations are possible in light of the above teaching. The exampleembodiments, as described above, were chosen and described in order tobest explain the principles of the disclosure and its practicalapplication to thereby enable others skilled in the art to best utilizethe disclosure in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the disclosure be defined by the claims appended hereto.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of capturing chemical species,comprising: contacting a metal-organic framework with a fluidcomposition including one or more of SO₂, CO₂, and H₂O, wherein themetal-organic framework is characterized by the chemical formulaNiNbOF₅(pyrazine)₂·x(Solv) or NiAlF₅(H₂O)(pyrazine)₂·x(solv), whereinthe metal-organic framework comprises a square grid pillared by aninorganic building block, wherein the square grid is Ni(pyrazine)₂ andthe inorganic building block is selected from [NbOF₅]²⁻ or[AlF₅(H₂O)]²⁻; and sorbing one or more of SO₂, CO₂, and H₂O from thefluid composition on the metal-organic framework.
 2. The method of claim1, wherein the fluid composition includes SO₂ at a concentration in therange of 25 ppm to 500 ppm.
 3. The method of claim 1, wherein the fluidcomposition includes SO₂ and CO₂, and SO₂ is preferentially sorbed overCO₂ on the metal-organic framework.
 4. The method of claim 1, whereinthe fluid composition includes SO₂ and CO₂, and SO₂ and CO₂ are bothsorbed on the metal-organic framework.
 5. The method of claim 4, whereinSO₂ and CO₂ are sorbed about simultaneously on the metal-organicframework.
 6. The method of claim 1, wherein the fluid compositionincludes CO₂ at a concentration in the range of 400 ppm to 5000 ppm. 7.The method of claim 1, wherein the fluid composition includes CO₂ andH₂O, and CO₂ is preferentially sorbed over H₂O on the metal-organicframework.
 8. The method of claim 1, wherein the fluid compositionincludes CO₂ and H₂O, and CO₂ and H₂O are both sorbed on themetal-organic framework.
 9. The method of claim 8, wherein the CO₂ andH₂O are sorbed about simultaneously on the metal-organic framework. 10.The method of claim 1, wherein the sorbing proceeds at about roomtemperature.
 11. A method of detecting one or more analytes, the methodcomprising: contacting a sensor to an environment containing one or moreof SO₂, CO₂, and H₂O, wherein the metal-organic framework ischaracterized by the chemical formula NiNbOF₅(pyrazine)₂·x(Solv) orNiAlF₅(H₂O)(pyrazine)₂·x(solv); wherein the sensor includes a layer of ametal-organic framework as a sensing layer, wherein the metal-organicframework comprises a square grid pillared by an inorganic buildingblock, wherein the square grid is Ni(pyrazine)₂ and the inorganicbuilding block is selected from [NbOF₅]²⁻ or [AlF₅(H₂O)]²⁻; anddetecting a presence of one or more of SO₂, CO₂, and H₂O in theenvironment using the sensor.
 12. The method of claim 11, wherein thedetecting proceeds at about room temperature.
 13. The method of claim11, wherein the detecting includes detecting SO₂ optionally in thepresence of H₂O.
 14. The method of claim 11, wherein the detectingincludes detecting between 25 ppm SO₂ to 500 ppm SO₂ in the environment.15. The method of claim 11, wherein the detecting includes detecting CO₂optionally in the presence of H₂O.
 16. The method of claim 11, whereinthe detecting includes detecting between 400 ppm of CO₂ and 5000 ppm ofCO₂ in the environment.
 17. The method of claim 11, wherein thedetecting includes detecting H₂O optionally in the presence of CO₂. 18.The method of claim 11, wherein the detecting includes detectingrelative humidity levels in the environment below about 40% RH and/orgreater than about 60% RH.
 19. The method of claim 11, wherein thesensor is a capacitive sensor comprising an interdigitated electrode,wherein the sensing layer is deposited on the interdigitated electrodeof the capacitive sensor, wherein the presence of one or more of SO₂,CO₂, and H₂O is detected by measuring a change in capacitance in thesensing layer.
 20. The method of claim 11, wherein the sensor is a QCMsensor comprising an electrode, wherein the sensing layer is depositedon the electrode of the QCM, wherein the presence of one or more of SO₂,CO₂, and H₂O is detected by measuring a change in resonance frequency inthe sensing layer.